It is known that certain enzymes are capable of hydrolyzing .alpha.-amino acids amides in an aqueous medium to .alpha.-amino acids. These so-called .alpha.-aminoacyl amidases (.alpha.-aminoacyl peptide hydrolase EC 3.4.11), also called aminopeptidases, show, like most enzymes, a very strong stereospecific activity and effect the hydrolysis of L-.alpha.-amino acid amides only. D-.alpha.-amino acid amides are either not hydrolyzed at all, or are hydrolyzed extremely slowly.
Aminoacyl amidases can be used for instance for the optical separation of amino acids, by contacting a DL-.alpha.-amino acid amide with the aminoacyl amidase and isolating the hydrolysis product L-.alpha.-amino acid and/or the D-.alpha.-amino acid amide; see Greenstein and Winitz, `Chemistry of the amino acids`, vol. 3, pp. 1778-1781 (New York 1961).
From U.S. Pat. No. 3,971,770 it is known that DL-amino acid amides can be separated into the corresponding D-amino acid amides and L-amino acids in the presence of an aminopeptidase preparation obtained from a culture of Pseudomonas putida.
However, in general the optical separation of .alpha.-amino acid amides containing an .alpha.-methyl group is not such a simple process. For instance, Tetrahedron Letters Vol. 23, No. 33, pp. 3335-3336 (1982) gives a description showing that the introduction of an .alpha.-methyl group into a good substrate for chymotrypsin, e.g. acetyl-L-tyrosine amide, makes the enzymatic hydrolysis rate of this substrate decrease by a factor of 10.sup.5. This decrease is believed to have to do with an unfavourable orientation of the bond to be split with respect to the active centre (steric hindrance due to the methyl group). Further, the applicant has found that separation of DL-2-amino-2,3-dimethylbutyramide does not take place in the presence of a preparation obtained from Pseudomonas putida and containing an aminoacyl amidase.